In a modern computing device, applications, utilities, and the operating system all run on, and share the processing power of, a central processing unit (“CPU”).
(Note 1: Computing devices now commonly parse out their work among multiple processors. For clarity's sake, the present discussion uses the term “CPU” to cover all single- and multiple-processor computing architectures.)
(Note 2: The phrase “applications, utilities, and the operating system” is shorthand for the set of all entities that may present information to a computer screen. Distinctions among these entities are, for the most part, of little import to the present discussion. Therefore, for clarity's sake, all of these entities are all usually called “applications” from here on unless a particular distinction is being drawn.)
When an “application” needs to present visual information to a user of a computing device, rather than directly issuing commands to the hardware that controls a display screen of that device, the application writes a series of graphics commands. These commands are sent to a graphics processing unit (“GPU”) which interprets the commands and then draws on the display screen.
Because the applications, utilities, and the operating system running on the device may all send graphics commands to the GPU, conflicts among the commands may arise. As a simple example, an application may command that a window be opened in a portion of the screen in which the operating system wishes to draw a background image (or “wallpaper”). The GPU knows the presentation hierarchy established among the applications (i.e., which one's display information should be “on top” and should thus overwrite that of the others) and resolves any conflicts accordingly. In the present example, the application is on top, so its window occludes a portion of the background wallpaper displayed by the operating system. Instead of occlusion, the GPU can be commanded to merge disparate images to make, for example, an “upper” image semi-transparent so that a “lower” image can be seen through it.